The present invention relates to printing on conductive substrate material and, more particularly, but not exclusively to printing on webs of materials such as aluminum using an electrophotographic printing machine.
Electrophotographic printing machines generally use a two-transfer system of printing in which an electrophotographic image is formed on a first drum using a laser beam shone onto a photoelectric material. An electrostatic image is formed in the photoelectric material by the laser beam and then ink is drawn into the electrostatic image. The image so formed is then transferred in a first transfer operation onto a blanket carried by an intermediate transfer drum, known as the ITM drum. A second transfer operation occurs when the image is transferred from the blanket onto the printing substrate which is held on a third drum, known as the impression drum.
Printing devices for separate sheets of paper are known that print colors by carrying out separate transfer operations for each color. That is to say they rotate the printing substrate over the impression drum several times, each time transferring the image per one color. When printing on web, multiple rotation of the drum for a single section of printing is not possible since the web is continuous. Therefore machines for printing on web use what are known as one-shot printing techniques, in which all of the printing images for all of the colors are gathered on the ITM drum and then transferred in a single rotation onto the web substrate.
Referring now to the drawings, FIG. 1 schematically illustrates a cross sectional view of an electrostatic printing assembly 1, according to the teaching of prior art. Apparatus 1 comprises an electrostatic drum 10 arranged for rotation about an axle 12. Drum 10 is typically formed with an imaging surface 16, e.g., a photoconductive surface. Surface 16 is typically of a cylindrical shape.
A charging unit 18, which can be a corotron, a scorotron, a roller charger or any other suitable charging unit known in the art, uniformly charges surface 16, for example, with positive charge.
Continued rotation of the drum 10 brings surface 16 into image receiving relationship with an exposing unit 20, which focuses one or more scanning laser beams onto surface 16 to scan a desired image. The laser beams selectively discharge surface 16 in the areas struck by light, thereby forming an electrostatic latent image. Usually, the desired image is discharged by the light while the background areas are left electrostatically charged. Thus, the latent image normally includes image areas at a first electrical potential and background areas at another electrical potential. Unit 20 may be a modulated laser beam scanning device, an optical focusing device or any other imaging device known in the art.
Continued rotation of the drum 10 brings imaging surface 16, now bearing the electrostatic latent image, into a developing unit 22, which typically comprises electrodes 24 operative to apply a liquid toner or ink on surface 16, so as to develop the electrostatic latent image. The liquid toner can comprise charged solid particulates dispersed in a carrier liquid. The solid particulates are typically charged to the same polarity of the photoconductor. Thus, due to electrostatic repulsion forces, ink particles adhere to areas on the photoconductor corresponding to the image regions, substantially without adhering to (developing) the background regions. In this manner a developed image is formed on surface 16.
Following application of liquid toner thereto, surface 16 typically passes through other rollers (not shown) which ensure that the ink surface is appropriate for transfer to ITM drum 40. A first ink transfer then occurs, in which the liquid image is transferred, typically via electrostatic attraction, from drum 10 to ITM drum 40, rotating in the opposite direction 41 of drum 10. In order for the first transfer to occur, an electrical bias is needed in the direction of image transfer. The drums are therefore generally biased negatively by a bias unit 44, so that a forward bias leads from electrostatic drum 10 to ITM drum 40.
Subsequently, the image experiences a second transfer, typically aided by heat and pressure, from ITM drum 40 to a substrate 42, which is supported by an impression drum 43.
Following the transfer of the liquid image to ITM drum 40, imaging surface 16 is cleaned to remove ink traces. Residual charge left on surface 16 can be removed, e.g., by flooding surface 16 with light from a lamp 58.
The electronic biasing provided by biasing unit 44 is problematic for printing on a conductive web substrate. Biasing unit 44 typically utilizes a voltage source-type power supply with a high voltage rating. The power supply is designed to fail when a high current is drawn, bringing about collapse of the bias path and thus failure of printing. Generally such failure only occurs in the rare event of a short circuit within the printing machine, however a problem arises when the web being printed is conductive, for example in the case of printing on aluminum sheet, say in the form of foil. In such a case the conductive substrate must contact the drum for the ink transfer to succeed. However, at the time the ink is being transferred to the substrate, ink is already being transferred to the ITM drum for the next operation, so as not to lose cycles within the machine. Thus a short circuit is formed through the printing substrate which is itself conductive, to earthed parts of the printing machine, giving rise to current leakage which is generally sufficient to collapse the bias and therefore stop the printing.
In the past a solution was found to allow the printing of conductive webs by isolating the conductive printing substrate from the rest of the machine. However such a solution is not practical in machines with sophisticated web feeding elements such as suction elements, since the suction elements are themselves made of conducting material and have to contact the web in order to work.
There is thus a widely recognized need for, and it would be highly advantageous to have, a means that would allow electrophotographic printing of conductive web substrates without being liable to current leakage.